This invention relates generally to ultrasonic systems used for the vaporization of tissue, and more particularly to a phased array ultrasonic system used for creating channels within organs by ablating tissue.
Cardiac ischemia is a condition in which oxygenated blood is reduced or cutoff to a section of the heart, usually as the result of cholesterol-laden plaque narrowing the coronary arteries and preventing blood flow. Untreated ischemia may lead to ischemic heart disease often with disabling angina. Angina is severe chest pain caused by insufficient oxygenated blood reaching the heart often during times of exercise or emotional stress. Untreated ischemic heart disease with its associated angina may lead to heart attacks and death or in somewhat less severe cases to a great reduction in the quality of life for the patient.
There are currently three treatments for the treatment of cardiac ischemic disease and angina. The first therapy is pharmacological. Drugs for reducing cholesterol and for managing the pain of the patient are administrated in conjunction with exercise in order to increase the amount of oxygenated blood reaching the cardiac tissue. The second treatment is balloon angioplasty. In this treatment a catheter is worked into the coronary arteries carrying with it a balloon, when the catheter reaches the portions of the coronary arteries that are clogged with plaque, the balloon is expanded compressing the plaque and opening the coronary artery wider in order to allow greater blood flow. The third current treatment is a coronary graft by-pass operation. The coronary by-pass operation is one where new arteries are grafted around the clogged coronary arteries creating new, unobstructed, blood passageways.
Recently a new treatment for cardiac ischemic disease has been developed called transmyocardial revascularization (TMR). In TMR tiny channels, approximately 1 mm in diameter, are drilled through the heart muscle to allow oxygenated blood from the left ventricle to flow through these channels into the damaged muscle tissue to bring oxygenated blood to those areas. Currently, TMR utilizes high powered lasers to drill these holes in the cardiac muscle. TMR is an invasive procedure since currently TMR techniques still require the chest to be opened sufficiently to visualize the heart. The laser is then used to drill holes from outside of the heart muscle through the entire heart muscle into the left ventricle. Although current studies show that the outer portion of the drilled channel does heal, more tissue is still damaged than is needed to bring oxygenated blood to the damaged tissues. In addition, the lasers used for TMR are expensive to obtain and to operate. In another method of laser TMR, the laser is threaded into the left ventricle and the channels are drilled into the wall directly. While this method does not destroy more tissue than necessary, it is still invasive, in that the laser is threaded through the circulatory system into the interior of the heart, and in particular to the left ventricle chamber of the heart, in order to be proximate to the target areas.
While TMR is a new procedure, the use of lasers to vaporize tissue, and the problems associated therewith, are not new. The present invention is superior to using lasers to vaporize tissue, since a laser can destroy more tissue than is necessary, or can perforate tissue causing additional complications, whereas the present invention is more easily controlled.
Other methods that exist to destroy tissue have other problems as well. For example, direct current cardiac tissue ablation requires a catheter to be inserted into the interior of the heart and 2,000 to 4,000 volts of electricity are applied to the target tissue over several milliseconds. In addition to being invasive, the severe muscle contractions which result, require the patient to be under general anesthesia.
RF and microwave ablation of cardiac tissue is invasive since it requires a catheter inserted into the interior of the heart. In addition the energy is difficult to focus and the size of the target tissue to be ablated is limited due to the lower energy available.
There are two methods currently used to deliver ultrasonic energy to target tissue. The first is to use a catheter with an ultrasonic transducer or transducer array on the tip. The catheter must be inserted into the interior of the heart and be in close proximity to the target tissue, due to the inability to narrowly focus the beam. Although phased array catheter probes have been discussed in the literature there are none commercially available. In addition, the size of the probe will limit the number of available phased array elements. The fewer the number of elements, the wider the main lobe of the antenna becomes. This will result in heating a wider area of tissue and hence cause the collateral destruction of healthy tissue during treatment. In addition more energy will be in the sidelobes of the phased array which will reduce the efficiency and possibly damage collateral tissue as well.
The second method of delivering ultrasonic energy is the use of an external ultrasonic transducer, having a phased array antenna in conjunction with a hydrophone array. The hydrophone is invasively placed to provide detection of the ultrasonic energy to determine its focus point. The hydrophone measurement provides the necessary feedback to adjust the beam focus properly so as to limit collateral tissue damage. However, the hydrophone array must be placed proximate to the target tissue so as to be effective.
Vaporization of tissue using ultrasound has not been described in the prior art. Although it has been known generally in the art that small cavities are formed in tissue during ultrasonic exposures with high power. However, there have not been any attempts to control this cavity and use it for tissue removal. In addition, the exposure parameters that cause desired, controlled tissue vaporization have not been known. There is a need for an apparatus to produce the energy required at the appropriate frequency to create tissue vaporization.
Therefore what is needed is an ultrasonic apparatus capable of producing sufficient pressure at the acoustic focus to vaporize tissue, and an ultrasound phased array with a feedback control system capable of measuring and controlling the power and phase from each individual array element such that TMR channels are created noninvasively within the myocardium by vaporizing tissue at the acoustic focus of the phased array. This will enable tissue to be vaporized without opening the patient""s body in order to actually see the tissue to be vaporized without having to invasively thread a catheter or hydrophone array into the patient""s body.
According to the invention, phased array ultrasonic devices are provided for use in vaporizing tissue non-invasively during medical treatment. The device includes a plurality of ultrasonic transducer elements which transmit ultrasonic waves each having a particular power and phase. The control of an individual ultrasonic transducer element to produce ultrasonic energy having a particular power and phase is needed to achieve constructive interference at the desired acoustic focus, and is achieved by a focusing means that is responsive to a feedback signal. This constructive interference creates high pressure amplitudes for vaporizing the target tissue at the focus. The individual ultrasonic transducer elements are supplied energy by a channel driver element that is responsive to the focusing means.
To achieve this desired acoustic focus, in one embodiment, the driver element is responsive to a focusing element and feedback means to properly drive the ultrasonic transducer elements. The focusing element comprises a controller that determines an operating parameter of the ultrasonic transducer element. The controller is responsive to the feedback signal and in the preferred embodiment determines the phase and power to be transmitted by each individual ultrasonic transducer and provides a control signal to each channel driver element of the corresponding ultrasonic transducer element. The controller in being responsive to the feedback signal, also provides the necessary control signals to the driver element to adjust the power and phase of each individual ultrasonic transducer element relative to other array elements in order to create the desired acoustic focus.
Each ultrasonic transducer has a portion of either the signal driving the ultrasonic transducer, or the ultrasonic energy emanating from the ultrasonic transducer, fedback so that its power is measured and its phase determined. These measurements are then provided in a feedback manner to the controller. The controller provides any necessary adjustment to the driver element for the ultrasonic transducer to correct any aberration from the desired operating parameter. In this way, the desired wave front of ultrasonic energy is generated and corrections of the wave front are made automatically to insure that only the desired target tissue volume is heated and vaporized. In addition, the phase and power measurements are made without the need for invasively including a hydrophone array within the patient""s body cavity.
In one aspect of the present invention, the driver element is a Class D/E switching amplifier. The Class D/E amplifier connects the ultrasonic transducer element to a power supply by switching at a particular frequency, and provides a high efficiency power transfer. In a preferred aspect of the present invention, high power MOSFET transistors are used as the switching elements in the Class D/E amplifiers.
In another aspect of the present invention, the controller comprises a power controller for controlling the level of DC power available to the driver element. The power controller is a switching DC/DC power regulator that provides power to the class D/E amplifier at a power level selected by the switching frequency. By varying the switching power regulator switching frequency, the power level to the Class D/E amplifier may be controlled and fine tuned according to the feedback signal and control inputs.
In yet another aspect the controller comprises a phase controller. The phase controller comprises a phase shifter that is the combination of a programmable delay chip and a counter. These chips combine to provide the total resolution of phase control of the transmitted ultrasonic wave. In addition, a phase locked loop is used as a phase detector and to ensure coherence between the phase shifter and the signal driving the power converter so that the proper wave is produced.
In another aspect of the present invention, a feedback means provides a feedback signal that is representative of the output signal. The feedback signal comprises a phase measurement and power measurement from either the input to, or the output from, a corresponding ultrasonic transducer. The feedback signal is fedback to a controller that will adjust a control signal provided to the mean for driving so as to correct the phase and power at the electrical signal provided to the corresponding ultrasonic transducer element.
A method is provided for vaporizing target tissue in which a plurality of ultrasonic transducers are focused so as to provide constructive interference within the target tissue. The ultrasonic beams are launched and produce fast tissue temperature rise and vaporizes the target tissue within the focal zone.
In another aspect, the invention features a method for forming a cavity in cardiac tissues in a subject. The methods include focusing an ultrasound beam on the target tissue, where the cavity is to be formed, thereby creating the cavity by vaporizing the target tissue.
In preferred embodiment the cavity is a channel connecting the left ventricle and tissue in the myocardium.
In preferred embodiment the method is noninvasive, i.e., the transducer is located outside the body of the subject.
In preferred embodiment a device described herein is used to form the cavity.
In preferred embodiment the energy delivered is sufficient to vaporize target tissue.
The present invention provides a safer and highly efficacious treatment than the prior art. The present invention is much safer than by-pass surgery in that the sternum is not split and the heart itself is not stopped for the duration of the operation. In addition, the present invention may provide a shorter recovery time and is not as costly a procedure to undertake.
While a preferred embodiment is described, it should be apparent that many modifications and variations are possible, all of which fall within the scope of the Detailed Description and Claims which follow.